The present invention deals with data storage in a magnetic disc data storage system. More particularly, the present invention relates to zone bit recording in a data storage system with write compensation.
A magnetic disc, such as one used in a computer disc drive, is a flat circular platter with a magnetic surface. Data is stored on the magnetic surface by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. Typically, the magnetically polarized portions are arranged in a plurality of radially concentric tracks on the magnetic surface to aid in location and read back of the data.
In order to read back the data recorded on the magnetic surface, a magnetic transducer supported by a read/write head assembly moves relative to the magnetic disc along a given track. The magnetic transducer generates an electrical signal (the "read signal"), which represents the states of polarization encountered along the track. Pulses in the read signal correspond to the magnetically polarized portions of the magnetic disc.
During processing of the read signal, the processing logic (i.e., the read chain logic) is configured to recover data from the read signal during a certain period in time known as a read window The read window represents the time during which the pulses in the read signal, which represent digital data stored on the magnetic disc, are valid. However, the actual window during which the pulses are valid can shift in time due to a variety of reasons including intersymbol interference (or "pulse crowding"). In other words, where pulses are written too close to one another on the magnetic disc, they interfere destructively causing the peaks in the read signal to shift in time relative to the read window. Where large intersymbol interference is encountered, the peaks can shift outside the read window causing erroneous data to be read back from the magnetic disc.
In order to overcome the peak shifts caused by intersymbol interference, many disc drive data storage systems are provided with a feature known as precompensation or write compensation. In such systems, the data pulses to be written to the magnetic disc are shifted in time, prior to being written, by an amount calculated to offset the peak shift due to intersymbol interference. The amount of pre-compensation needed can be measured and implemented in a variety of known manners.
Notwithstanding the fact that intersymbol interference is a problem in disc drive systems, it is desirable to maximize the amount of data that can be stored on a disc in order to save space and reduce the number of discs needed to store a particular amount of data. The outer tracks on a magnetic disc are longer than the inner tracks Thus, data stored on a magnetic disc at constant frequency using constant angular velocity results in a significant decrease in data density on the outer tracks Consequently, storage space is wasted on all but the innermost track when using a fixed frequency, fixed angular velocity method of data storage.
As discussed in the Bremmer et al U.S. Pat. No. 4,799,112, the amount of data stored by a magnetic data storage system can be increased by grouping the tracks on the magnetic discs into frequency zones. All the tracks in each zone are written at a given zone frequency assigned to the particular zone in which the track is located The frequency at which data is written differs from zone to zone along the surface of the disc. Bremmer et al disclose a circuit for implementing such an approach to data storage optimization. Bremmer et al further disclose a method for selecting the zone frequencies and track-to-zone assignments in a way that results in a substantially constant error rate in the writing of data to, and the subsequent reading of data from all the tracks on the magnetic disc.
In the implementation taught by Bremmer et al, an error rate for each track at each of the selected zone frequencies is determined by measuring a magnetic characteristic of the disc. In particular, Bremmer et al measure the shape of an electrical pulse produced by the read/write head as the head passes a single magnetic reversal on the disc surface. Bremmer et al use a mathematical model to determine the resolution of a series of such pulses The error rate, associated with the calculated resolution, is controlled by assigning tracks to frequency zones for which the calculated resolution meets a selected threshold value.
While Bremmer et al teach an effective way of increasing data storage on magnetic discs, they do not implement the technique of providing write compensation to accommodate for intersymbol interference.